1. Field of the Invention
This invention relates to the field of data storage devices, such as hard disk drives with thin film magnetic disks. More particularly, the present invention relates to a configuration of, and a method for fabricating, a thin film medium, which can be used for magnetic recording applications requiring ultra high density storage of data.
2. Description of the Relevant Art
The magnetic thin film media in a background art hard disk drive consists of a substrate (Glass, Al alloy etc.), an underlayer of Cr or Cr alloy, a magnetic layer of Co-alloy, and a protective overcoat of carbon and lubricant. The data bits are stored as the transition of magnetization of a group of tiny magnetic grains. The signal to noise ratio (SNR) is roughly proportional to the number of grains.
In accordance with the background art, the way to increase an area density of stored data in a magnetic recording medium is to reduce the grain size and thickness. The reduction of grain size leads to sharper transitions and a larger SNR. The reduction of the remnant moment-thickness product (Mrxcex4) leads to reduced demagnetizing fields and lower noise.
Unfortunately, the reductions in grain size and thickness lead to a reduction in the available energy (KuV) to store written bits (where Ku is the anisotropy constant, and V is the volume of the grain). At room temperature, the presence of thermal energy increases the possibility of magnetization decay, if KuV is small. The ratio, KuV/kBT, should be about 60, if the data bits are to remain thermally stable. In the conventional single layered magnetic media, this limiting value is achieved at a recording density of 40 Gb/in2.
To surpass this limit value, alternative techniques and/or materials have been proposed. Utilization of antiferromagnetic underlayers is one of the ways proposed to overcome the thermal instability issue. U.S. Pat. No. 6,020,060, issued to Yoshida et al., discloses using a bcc structured antiferromagnetic layer.
Further, Abarra et al. and Fullerton et al. have proposed in journals and conferences, the addition of one or two stabilizing layers made of a magnetic material, coupled antiferromagnetically to the magnetic recording layer. See E. N. Abarra, H. Sato, A. Inomata, I. Okamoto, and Y. Mizoshita, AA-06, presented at the Intermag 2000 Conference, Toronto, April 2000. Also see, E. N. Abarra, A. Inomata, H. Sato, I. Okamoto, and Y. Mizoshita, xe2x80x9cLongitudinal magnetic reording media with thermal stabilization layers,xe2x80x9d Applied Physics Letters, Vol. 77, No. 16, 2000, pp. 2581. Also see, Eric E. Fullerton, D. T. Margulies, M. E. Schabes, M. Carey, B. Gurney, A. Moser, M. Best, G. Zeltzer, K. Rubin, H. Rosen, and M. Doerner, xe2x80x9cAntiferromagnetically coupled magnetic media layers for thermally stable high-density recording,xe2x80x9d Applied Physics Letters, Vol. 77, No. 23, 2000, pp.3806.
According to the configuration proposed by Abarra et al., two stabilizing ferromagnetic layers are deposited below the main recording layer to improve the thermal stability. Thermal stability is increased because of the increase of grain volume, as well as, due to the antiferromagnetic coupling at two interfaces. Unfortunately, the Mrxcex4 is not greatly reduced, because the moment of two ferromagnetic layers is reduced by the moment of only one ferromagnetic layer. Therefore, the total Mrxcex4 remains relatively large.
According to the configuration proposed by Fullerton et al., only one stabilizing layer is deposited below the main recording layer. In this configuration, the Mrxcex4 reduction is larger than the configuration of Abarra et al. However, the increase of thermal stability is not very large, because the increase in the grain volume is not large, and also there is only one antiferromagnetically coupled interface.
U.S. Pat. No. 6,077,586, issued to Bian et al., discloses a structure similar to that of Fullerton et al. However, the structure of Bian et al. is not based on antiferromagnetic coupling, because Bian et al.""s spacer layer is more than 1 nm in thickness.
FIG. 1 is a cross sectional view showing a configuration of a thin film magnetic disk, in accordance with a first embodiment of the background art. The configuration of FIG. 1 is used by the Fujitsu corporation. The configuration includes a first ferromagnetic layer L1 having a first thickness t1, a second ferromagnetic layer L2 having a second thickness t2, and a third ferromagnetic layer L3 having a third thickness t3. The first layer L1 is the main recording layer. Non-ferromagnetic spacer layers separate the second ferromagnetic layer L2 from the first and third ferromagnetic layers L1 and L3. An intermediate layer, underlayer and substrate reside beside the first ferromagnetic layer L1. Further, an overcoat and lubricate reside beside the third ferromagnetic layer L3. In this configuration, Mr is defined as the remnant moment. Even if full cancellation of moments between the layers L1, L2, L3, at remanence is assumed, the total Mrxcex4 is given by Mr (t1xe2x88x92t2+t3).
FIG. 2 is a cross sectional view showing a configuration of a thin film magnetic disk, in accordance with a second embodiment of the background art. The configuration of FIG. 2 is used by the IBM corporation. The configuration includes a first ferromagnetic layer L1 and a second ferromagnetic layer L2. The first ferromagnetic layer L1 has a thickness of t1, and the second ferromagnetic layer L2 has a thickness of t2. A non-ferromagnetic spacer layer reside between the first and second ferromagnetic layers L1 and L2. Again, the thin film magnetic disk includes a substrate, an underlayer, an intermediate layer, an overcoat and a lubricant, as discribed in conjunction with FIG. 1. In this configuration, the total Mrxcex4 is given by Mr (t1xe2x88x92t2), if full cancellation of the moments between the layers at remanence is assumed. Thus, in the configurations of FIGS. 1 and 2, the reduction of the Mrxcex4 comes from only one stabilizing layer, namely layer L2.
In the background art, a reduction in Mrxcex4 comes from only one of the stabilizing layers, namely the layer L2 in FIGS. 1 and 2. The present invention appreciates that if the reduction of Mrxcex4 comes from two stabilizing layers, the recording media will be less noisy, i.e. a higher SNR can be achieved. Also, the present invention appreciates that if there are two or more anti-ferromagnetically coupled interfaces, the data will be more thermally stable. Therefore, the total number of ferromagnetic layers is three or more. The present invention proves a thin film magnetic media for storing data having a relatively increased thermal stability and a relatively reduced Mrxcex4 in comparison to the background art configurations.
Another object of the present invention is to provide a fabrication method for a high-density longitudinal magnetic recording medium, which can support an ultra high recording area density relative to the background art configurations.
Yet another object of the present invention is to provide a medium having magnetic layers disposed adjacent to non-ferromagnetic spacer layers, such as Ru. The antiparallel coupling between the layers cause a reduction in the Mrxcex4. In such a configuration, the Mrxcex4 reduction comes from two layers. Such a media is suitable for very high-density recording.
It is a further object of the present invention to provide a magnetic recording medium with three ferromagnetic layers. Two non-ferromagnetic spacer layers separate the three ferromagnetic layers from each other. Out of the three ferromagnetic layers, the middle layer is thicker than the other two layers and is the main recording layer. The Mrxcex4 reduction comes from the top and bottom ferromagnetic layers and so, the Mrxcex4 reduction is larger. Because of two antiferromagnetically-coupled interfaces and a larger grain volume the thermal stability is also larger.
Other objects and further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.